A direct conversion receiver, known alternatively, as a zero intermediate frequency receiver (ZIF receiver) has a homodyne architecture and converts (transposes) the received signal, for example, a radio frequency signal, directly into baseband, i.e. around the 0 Hertz frequency. The advantages of such an architecture with the homodyne structure are the elimination of both the image processing and the intermediate frequency filters. For this reason, such a receiver is easier to integrate than a heterodyne receiver in which the conversion into baseband is carried out by way of one or more intermediate transpositions at intermediate frequencies.
However, a homodyne architecture may have drawbacks, which are notably the presence in the transposed signal of a DC offset, flicker noise, a greater sensitivity to leaks from the local oscillator generating the frequency transposition signal, and also to the mismatches between the two quadrature channels I and Q of the receiver system.
The DC offset may be a major drawback in the direct conversion architecture. The phenomenon, in fact, comprises of the appearance of a DC offset around the 0 Hertz frequency, which then leads to degradation of the performance of the receiver. It may important to eliminate this offset in the analog part of the receiver system, before the analog-digital conversion, in order to avoid saturating the digital processing elements in the baseband. This is because such saturation causes significant distortions of the signal.
Two types of DC offset can be differentiated, namely, a static DC offset and a dynamic DC offset, i.e. DC offset varies over time. From amongst the phenomena causing the static DC offset, several that may be mentioned are: the self-mixing of the local oscillator (the auto-transposition of the transposition signal supplied by this local oscillator) resulting from the leaks of the transposition signal into the RE signal input of the oscillator, the unbalancing between the two differential channels of the mixer, and the mismatch between the I and Q channels.
From amongst the phenomena causing the dynamic DC offset, several that may be mentioned are: the leakage from the mixer towards the antenna which may be modified depending on the environment and picked up, here again causing a self-mixing, and the RF signal leaks towards the oscillator and the even-order harmonic distortion. As indicated hereinabove, the self-mixing results from leaks occurring in the transposition signal generated by the local oscillator towards the signal input of the mixer or else towards the input of the low-noise amplifier generally disposed upstream of this mixer (and which may therefore amplify this leak). The transposition signal delivered by the local oscillator is then mixed with itself (auto-transposition), thus creating a DC component or DC offset. The level of this offset depends on the leaks and on the phase difference between the transposition signal and the leaks. The level of the resulting DC offset may then be higher than that of the desired signal. This offset is a static DC offset.
Furthermore, since the transposition signal is situated within the passband of the pre-selection filter disposed just after the antenna, it may perfectly well be envisaged that the leaks reach as far as the antenna where they are subsequently returned. In that case, the leaks from the local oscillator can interfere with other receivers in the wireless communications system, but also be reflected on external objects. Given that the environment can change, the leakage signal may change in amplitude and phase, then creating a dynamic DC offset.
Several methods for the suppression of DC noise signals exist. Amongst these may be mentioned the method comprising placing a capacitive high-pass filter in the receiver channel downstream of the frequency transposition stage in order to cut off the DC signal. However, such a system may be slow to tune and poses problems when establishing communications. Furthermore, the signal may be cut off at very low frequencies, which causes a degradation of the signal (loss of data).
Another approach comprises creating a negative feedback loop in the radio frequency channel, which then forms a negative feedback at the frequencies close to that of the DC offset, thus allowing it to be filtered. Another possible approach that may be mentioned is the extraction of the DC offset from the useful signal in order to be able to eliminate it in real-time by using a master-slave architecture comprising, notably, using a second channel called the “slave” and having the same characteristics as the main channel. However, such an approach may only allow the static DC offsets to be eliminated and not the dynamic DC offsets. Moreover, since all the components are doubled up, the power consumption of the circuit increases.
Another approach comprises placing a Miller integrator in negative feedback downstream of the frequency transposition stage of each of the channels I and Q in order to eliminate the offset. This loop is constructed around a variable-gain amplifier. The DC voltage difference at the output of each mixer is sensed by the Miller integrator, which has the effect of making the differential voltage at the output of the mixer vary. However, such an approach may have a drawback for the communications systems of the CDMA or WCDMA (Wide Band Code Division Multiple Access) type in which QPSK modulation is used. The reason for this is that the power spectrum of the signal comprises a component at 0 Hertz.
Now, such a approach with Miller integrator in negative feedback mode performs a blind suppression of the DC signal, in other words, it does not differentiate between the offset of the useful signal and the noise-signal offset. As a result, the final power spectrum of the signal may virtually no longer have any component around 0 Hertz. Moreover, the Miller integrator also degrades the signal close to 0 Hertz. Consequently, this may result in the DC offset noise signal being eliminated but at the expense of the quality of the received signal leading to many decoding errors and to a poor quality of reception.